Digital prefilter for clutter attenuation in MTI radars

ABSTRACT

In a digital moving target indicator (MTI) employing digital filters for rejecting low frequency clutter signals having magnitudes which are 20 to 60 db greater than the moving target return signal, the dynamic range and resolution accuracy requirement of an analog-to-digital (A/D) converter, used to convert the return radar analog video signal to a plurality of digital signal equivalents for presentation to the digital filters, is reduced from that normally required by prefiltering the return video signal through a prefilter network. The digital prefilter utilizes: a low resolution accuracy (low order bit capacity) analog-to-digital (A/D) converter to convert the return analog video signal (comprising both low frequency clutter and high frequency moving target component signals) to a digital signal, a low-pass digital filter to pass only the low frequency clutter signal, a digital-to-analog converter to reconvert the digital clutter signal to an analog clutter signal which is substantially equivalent to the clutter component signal value of the analog video signal, and a summing circuit which provides an analog sum signal representative of the amplitude difference between the analog clutter signal and the return analog video signal, to provide a video signal with a greatly enhanced moving target to clutter signal ratio. The sum signal is presented to the system A/D converter, permitting the use of a low resolution system A/D converter having a substantially reduced dynamic range and lower cost, the system A/D converter providing the digital equivalent signal of the enhanced return video for further processing in the digital MTI filters.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to radar systems having digital moving targetindicators, and more particularly to prefiltering of the return radaranalog video signal prior to presentation to the MTI digital filters.

2. Description of the Prior Art

The use of moving target indicator (MTI) radars to detect moving targetswithin a scanned sector by detecting the change in amplitude, or phaseof the Doppler signal frequency of such targets, is well known in theart. In such systems the reflected radar energy is received from targetsin the sector and processed to provide an analog video signal havingcomponent Doppler frequencies dependent upon the nature of the target.These signal components are described generally as consisting of smallamplitude, high frequency signals from moving targets, and largeamplitude, low frequency signals from relatively stationary targets. TheMTI radar systems are capable of discriminating between small, varyingamplitude signals (denoting moving targets) and relatively fixedamplitude return signals indicative of stationary targets, and generallyreferred to as clutter. The clutter component signal represents themajor portion of the return radar analog video signal, such that movingtarget signals are masked by a clutter component signal which may be inthe order of 20 to 60 db greater. The effectiveness of the MTI systemdepends upon its ability to filter out the unwanted clutter portion of areturn signal so that moving target signals of the lowest order may bedetected.

A number of clutter filtering techniques are known in the art. Theseinclude delay line cancellers which delay the return video signals ofeach main bang pulse by one pulse repetition period (PRP) and cancel thefixed amplitude components of a subsequent PRP return to provide movingtarget indication. Other systems, commonly known as range-gated movingtarget indicators (RG-MTI), quantize the return signal portion of thePRP into range intervals or bins, providing Doppler, or band passfilters within each range bin to filter out the lower frequency cluttercomponent and pass only the higher frequency moving target components.In the past, the delay line cancellers, or range-gated band pass filtershave been of the analog type consisting of passive impedences (such ascapacitors, resistors and inductors, whether alone or distributed), andactive networks including amplifiers. However, such devices usually havebroad, rather than sharp, roll off characteristics, and inherently areextremely difficult and costly to make adaptable. Some of the morerecent MTI radar systems employ digital filters, which are implementedwith digital signal handling techniques and the use of Z transformequations. Such digital filters can produce very sharp roll offcharacteristics and are easily modified to vary the attenuationresponse, however, they are expensive and complex. One of the singlemost expensive components in a digital filter MTI system is that of theanalog-to-digital (A/D) converter which converts the return radar analogvideo signal into a digital signal equivalent for presentation to thedigital MTI filter and detection circuitry (generally of the range-gatedband pass type). Due to the extremely low moving target-to-cluttersignal ratios (typically minus 20 to minus 60 db) the dynamic range, ortotal bit capacity of the A/D converters must be high to insure aconverter resolution accuracy which is capable of providing amplitudediscrimination between the moving target signal component and theclutter component, to preserve the target signal component in thedigital word. These A/D converters, which typically comprise twelvebits, or more, are one of the single most expensive components in thesystem. Therefore, the cost and the ability to provide a sufficientlyhigh dynamic range A/D converter, present the chief problem andperformance limitation of the present state of the art MTI systemsemploying digital filters.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a reduced cost digitalmoving target indicator (MTI) radar system with an improvedtarget-to-clutter signal ratio. Another object of the present inventionis to provide a low cost clutter filter circuit for use in existingdigital MTI radar systems.

According to the present invention, a digital MTI radar includes atiming signal oscillator, a signal source for providing pulsed radarenergy for illuminating a spatial sector at a pulse repetition periodcontrolled by the timing signal, and for receiving, during a non-pulseportion of the pulse repetition period, reflected energy from sectortargets to provide an analog video signal having a high frequency targetsignal component representative of moving targets and a low frequencyclutter signal component representative of relatively stationary targetsand characterized by a given target-to-clutter signal ratio, prefiltercircuitry for reducing the low frequency clutter signal component toprovide a filtered analog video signal with a target-to-clutter signalratio which is substantially greater than that of the signal sourcevideo signal, a first analog-to-digital (A/D) converter for providing aplurality of digital video signals representative of the filtered analogvideo signal amplitude coincident in a plurality of successive timeintervals provided by quantization of the pulse repetition period by thetiming signal, the analog-to-digital converter having a resolutionaccuracy for providing amplitude discrimination between the targetsignal component and the reduced clutter signal component, theresolution accuracy being substantially less than that required toprovide equal accuracy amplitude discrimination of the target signalcomponent of the signal source video signal, the MTI radar systemfurther including filter and detection circuitry for detecting the highfrequency target signal component of the digital video signal and forproviding in response thereto, a signal manifestation representative ofa moving target.

In further accordance with the present invention, the prefiltercircuitry includes a second A/D converter for providing a plurality ofdigital video signals representative of the analog video signalamplitude coincident in each of the plurality of successive timeintervals of the pulse repetition period, the converter having aresolution accuracy which is less than that required to provide accurateamplitude discrimination between the target signal and clutter signalcomponents of the analog video signal, a digital low-pass frequencyfilter circuit for attenuating the high frequency target signalcomponent of the digital video signals to provide a plurality offiltered digital signals representative of the low frequency cluttercomponent amplitude of the analog video signal, a digital-to-analogconverter for providing an analog signal equivalent of the plurality offiltered digital signals which is representative of a substantialportion of the clutter component amplitude of the signal source analogvideo signal, and summing circuitry for providing to the firstanalog-to-digital converter a sum signal representative of thedifference amplitude between the analog signal equivalent and the signalsource analog video signal.

In still further accord with the present invention, the digital low-passfilter circuit includes a first multiplier circuit for providing inresponse to the plurality of digital video signals, a plurality offilter input signals representative of the product of the digital videosignals and a scaling constant less than unity, a full adder circuit foradding the input digital signals presented at one input of the adder toa plurality of feedback digital signals presented at another input ofthe adder to provide a plurality of sum digital signals, the additionoccurring for input and feedback signals coincident in a same timeinterval, each sum digital signal including a plurality of digital bitsprovided on a plurality of digital bit lines to the digital-to-analogcircuit, a plurality of serial shift registers, each connected to adifferent one of the digital bit lines, each register providing, inresponse to successive time intervals, sequential shifting and storageof the plurality of successive digital bits corresponding to theplurality of digital sum signals, the plurality of shift registers, incombination, storing the plurality of sum signals in each pulserepetition period and presenting the digital sum signals, one duringeach time interval, on a plurality of signal lines after a delay of onepulse repetition period and a second multiplier circuit for providing aplurality of feedback signals representative of the product of each ofthe delayed sum digital signals from the shift register outputs and asecond scaling constant whose value is equal to the difference betweenthe first constant value and unity, and for presenting the plurality offeedback signals to the other input of the full adder circuit.

The prefilter circuitry of the present invention, is adaptable toexisting MTI systems, and by providing a substantial enhancement of thetarget-to-clutter signal ratio of the analog video signal from thesignal source, permits a reduction in resolution accuracy of theexisting system A/D converter. The component cost of the prefiltercircuit including the added low resolution A/D converter, beingsignificantly less than that of a high resolution (high bit capacity)A/D converter required by existing digital MTI systems to provide equalamplitude discrimination of the target signal in the high clutterbackground. Similarly, for systems requiring a higher degree of movingtarget detection accuracy, a design flexibility is provided in allowingan increase in resolution accuracy of the prefilter A/D converter,and/or the system A/D converter, to permit higher detection accuracy ata cost which is equal to, or less than, existing system cost. Otherobjects, features and advantages of the present invention will becomemore apparent in the light of the following detailed description of apreferred embodiment thereof, as illustrated in the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a system block diagram of an embodiment of the presentinvention;

FIG. 2 is a schematic diagram of a prefilter circuit used in theembodiment of FIG. 1; and

FIG. 3 is an illustration of a received signal used in conjunction withthe operation of the embodiment of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, in an exemplary embodiment of a moving targetindicator (MTI) radar system according to the present invention, atransmitter 10 provides radar main bang pulses with a pulse repetitionperiod (PRP) dependent on a timing signal from a clock 11, which isprovided through a line 12 to the transmitter. The pulses are presentedthrough a wave guide 13, a transmit/receive (T/R) switch 14, and a waveguide 16 to an antenna 18 for propagation into a spatial sector.Reflected radar energy from targets in the scanned sector is received bythe antenna 18 and conducted through the wave guide 16, T/R switch 14,and a wave guide 20 to a receiver 22. The transmitter 10 and receiver 22may be operated in either the coherent or non-coherent mode, and theDoppler frequency signal components of the return radar are provided asan analog video signal at the output of the receiver 22 on a line 24.

As described hereinbefore, the analog video signal includes both highfrequency components from moving targets and a low frequency componentfrom relatively stationary targets, or clutter. The clutter componentsignal, representative of the background terrain against which a movingtarget must be detected, comprises the majority of the analog videosignal amplitude. In a non-coherent radar system, a moving target isdetected by sensing the changing amplitude of the radar return signal insuccessive pulse repetition periods, resulting from the change inDoppler frequency as the target moves away from, or towards thetransmitter. Since the typical moving target-to-clutter signal ratio ison the order of minus 20 to minus 60 db, the moving target signalamplitude may only represent one-thousandth of the clutter amplitude inany single pulse repetition period. One such return in a single pulserepetition period is shown in FIG. 3, wherein the amplitude of thereturn target component 25 is on the order of 10 millivolts, as comparedto an average clutter amplitude on the order of 10 volts. The timeintervals t₁ through t₄ represent a portion of a plurality of timeintervals provided by a quantization of the non-pulse portion of thepulse repetition period by the line 12 timing signal, and represent thetime intervals during which the analog video signal is sampled andconverted into successive digital signals by a system A/D converter, asdescribed in detail hereinafter. In a range-gated MTI, the timeintervals are representative also of the system range bins. Typicallythe moving target amplitude appears in more than one of the timeintervals, and the A/D converter (with a conversion time of t_(c))samples the amplitude only once during each of the successive intervals,as indicated by the sample points 26, 27. The sample at 26 provides a 10volt amplitude and the sample at 27 provides a 10.005 volt amplitude,resulting in a differential sampled amplitude of 5 millivoltsrepresenting the return target signal. Therefore, the A/D converter mustbe capable of detecting 5 millivolts out of approximately 10 voltsresulting in a required minimum resolution accuracy of 0.05 percent. Thequantization error, which is a measure of an A/D converter resolutionaccuracy, is defined as being equal to ± 1/2 LSB. If the maximumamplitude range of the analog video signal is twelve volts and the LSBis equal to 5 millivolts, the A/D converter requires a scaling, or worddecimal value (full scale value) of 12v ÷ 0.005v = 2400, whichrepresents a binary word length in excess of 11 bits. Consideration ofother circuit tolerances would therefore lead to the selection of atwelve bit, or 4095 word decimal value, where the LSB = 2.93 millivolts,and the quantization error is approximately ± 1.5 millivolts. Since -60db represents a minimum typical value of moving target-to-clutter signalratio, as may be appreciated, smaller signal ratios result in relativelysmaller target amplitudes and higher resolution accuracy requirementsfor the system A/D converter. A/D converter bit capacities of 13 and 14bits are not uncommmon in present digital MTI systems, resulting in adisproportionate cost factor being allocated for the A/D converter ascompared to the remaining digital circuitry.

Referring again to FIG. 1, in a typical prior art digital MTI system,the analog signals on the line 24 are presented directly to a system(A/D) converter, such as the A/D converter 28, which provides aplurality of digital video signals, each equivalent to the amplitude ofthe analog video signal in a different one of the plurality of timeintervals of the PRP, and presents these digital signals through a setof lines 29 to a filter and detection network 30. The digital videosignal for each time interval is clocked into the MTI filter network 30on successive line 12 clock cycles. The MTI filter and detection network30 is a digital network which may comprise either a plurality of rangegated filter networks (the number of range gated networks required beingdependent upon the range coverage, and the main bang pulse width), or ofa delay line canceller type network, both of which are well known in theart, and which filter the composite clutter plus moving target returnsignal to remove the clutter component, and detect the moving targetcomponent. The output of the filter and detector network is providedthrough a set of lines 32 to a display processor 34, which provides anindication of the moving target on any suitable type display as may berequired in an individual MTI system.

In accordance with the present invention, the requirement for a largebit capacity, or high resolution accuracy for the A/D converter 28 iseliminated by reducing the clutter component amplitude of the analogvideo signal in a prefilter network 36, thereby enhancing the movingtarget-to-clutter signal ratio prior to conversion in the A/D converter28.

The prefilter 36 includes an A/D converter 38 which receives the returnradar analog video signals on the line 24 from each PRP, and provides aplurality of low resolution equivalent, digital video signals on a setof lines 40, for each PRP time interval. The resolution accuracy of theA/D converter 38 is less than that required to provide amplitudediscrimination between the moving target signal component and theclutter signal component, and therefore, provides a digital video signalwith a word decimal value substantially smaller than that required forconverting the entire analog video signal at a resolution sufficient fordetecting a moving target in the predominantly clutter return. Thedigital video signals on the lines 40 are presented to a low-pass filter42, which passes only the lower frequency clutter signal components (thefilter cut-off frequency being selected to be greater than the maximumfrequency of the clutter signal component and less than the minimummoving target frequency) as described in detail hereinafter. The filter42 presents the digital clutter signal component through a set of lines44 to a digital-to-analog (D/A) converter 46. The D/A converter 46reconverts the digital representation of the substantially pure cluttersignal component on the lines 44 into an analog signal, which isprovided at the output of the converter on the line 48. The resolutionaccuracy of the D/A converter is relatively high so that it may providean analog signal which is substantially equal in magnitude to thatportion of the clutter signal provided by the low resolution A/Dconverter 38. However, as may be known, the cost of providing a highresolution D/A converter is far less than that of an equal resolutionA/D converter. The analog clutter signal component on the line 48 ispresented to a summing junction 50, which receives at a second input,the return radar analog video signal on the lines 24. The clutter signalcomponent is subtracted from the composite clutter plus moving targetcomponents of the line 24 video signal in the summing junction 50, andan analog sum signal representative of the difference amplitude betweenthe two signals is presented through a line 52 to the A/D converter 28.The analog sum signal on the line 52 has a substantially reduced cluttercomponent and a greatly enhanced moving target-to-clutter signal ratio,permitting a reduction in the bit capacity and consequently theresolution accuracy of the A/D converter 28 by as much as a factor ofone-half.

Referring now to FIG. 2, the analog video signal on the line 24 ispresented to the A/D converter 38 which provides a plurality of digitalvideo signals in response thereto on the lines 40. As statedhereinbefore, the resolution of the A/D converter 38 is substantiallyless than that required for amplitude discrimination or detection of themoving target signal from the clutter signal. The lines 40,corresponding in number to the number of output bit lines of the A/Dconverter 36, are presented to the low pass filter 42 (FIG. 1) at oneinput of a multiplier 54, comprising a number of individual multipliersof a type well known in the art such as the Fairchild 9344. Themultiplier 54 receives a digital signal K₁ at a second input, whichrepresents a scaling constant with a value less than unity, and providesan input digital signal, representative of the product of the digitalvideo signal and the constant K₁, through a set of lines 56 to one inputof a full adder 58, such as the Fairchild 9383. As may be appreciated,the scaled input digital signals may be provided through any combinationof processing which provides an equivalent product value. The full addercombines the plurality of input digital signals on the lines 56 with aplurality of feedback digital signals appearing on a set of lines 60 ata second input to the adder. As described hereinafter, the feedbackdigital signals represent the product of each of the plurality of inputdigital signals from the immediately preceding pulse repetition period,and a feedback scaling constant value less than unity. The output of thefull adder 58, a plurality of digital sum signals, is presented throughthe lines 44, comprising a plurality of digital bit lines, to the D/Aconverter 46, and shift registers 62 through 65. The shift registers,one for each digital bit line within the lines 44, are of the serial in,serial out type, such as the Texas Instrument SN7491A, and include anumber of such shift registers in series to provide a plurality of bitscorresponding to the number of time intervals provided within thenon-pulse portion of the PRP. The registers 62-65 are also presentedwith the clock signal on the line 12 which controls the serial shiftingof the individual sum signal bits through the registers. The pluralityof sum digital signals from the shift registers 62-65 are providedthrough lines 66 through 69 respectively, to a second multiplier 70,similar to the multiplier 54. The multiplier 70 receives at a secondinput a digital signal K₂, which represents a feedback scaling constantwhose value is less than unity, and provides a plurality of digitalfeedback signals, representative of the product of the sum signals andthe constant K₂, to the second input of the full adder 58. The sum ofthe constants K₁ and K₂ is equal to unity, and the clock signal on theline 12 is presented to the A/D converter 38, the multiplier 54, thefull adder 58, the shift registers 62-65, the multiplier 70, and the D/Aconverter 46, to provide synchronization of the prefilter 36 with thetransmitter 10 and filter and detection network 30.

In operation, the clock signal on the line 12 controls the sampleincrement of the A/D converter 38 in each PRP time interval, whichprovides the digital video signal equivalent of the analog video signalamplitude appearing in each time interval, on the lines 40. The lines 40including a plurality of lines equal to the bit capacity of theconverter 38, and extending from the least significant bit (LSB) to themost significant bit (MSB). The number of bits is equal to the selectedresolution of the A/D converter as described hereinbefore. Themultiplier 54 provides the product (input digital signal) of eachdigital video signal and K₁ through the lines 56 to the full adder 58.The full adder 58 adds each of the input signals on the lines 56 withthat one of the plurality of feedback signals on the lines 60 whichappears in the same time interval, and provides the sum digital signalon the digital bit lines 44. A new set of digital sum signals isprovided on the plurality of digital bit lines for each PRP timeinterval. The individual bits of the sum signal on each of theindividual bit lines are clocked through a respective one of the shiftregisters 62-65 on successive line 12 clock cycles, such that at the endof one PRP each of the shift registers has stored a plurality of serialbits, each representing one bit in the corresponding plurality ofdigital sum signals provided in that PRP, the combination of theregisters 62-65 providing serial storage of the sum signals provided inthe PRP. With the beginning of the next PRP, the sum signal data storedin the shift registers 62-65 is clocked out, one digital signal at atime beginning with the sum signal provided from the first interval ofthe preceding PRP, as the digital signals corresponding to the timeintervals of the second PRP are received at the multiplier 54. The shiftregisters function as a first in/first out memory storage for successivePRP cycles, such that as the first time interval sum signal of a secondPRP appears on the lines 40, the first time interval sum signal of theimmediately preceding PRP is clocked onto the lines 66-69, and ismultiplied in the multiplier 70 by the scaling constant K₂. This scaledsignal, or feedback signal, is presented through the lines 60 to thefull adder 58, and added to the first time interval input signalprovided by the multiplier 54 from the second PRP. The sum of these twofirst time interval signals of the two successive sweeps is presented onthe lines 44 to the D/A converter 46, and the shift registers 62-65,which clock this sum signal, one clock interval at a time, through theregisters to repeat the cycle for successive pulse repetition periods.The successive plurality of sum signals appearing on the lines 44 fromeach PRP, represent only the lower frequency clutter component of thedigital video signals provided by the A/D converter 38. This may beshown by allowing S to represent the total number of digital videosignals appearing on the lines 40 in any one pulse repetition period.Then, the total digital sum signal (S_(T)) appearing on the lines 44 onsuccessive pulse repetition periods is described as follows:

    ______________________________________                                        PRP               SUM SIGNAL                                                  ______________________________________                                        1st       S.sub.T =S.sub.1 K.sub.1 K.sub.2                                    2nd       S.sub.T =S.sub.2 K.sub.1 K.sub.2 +S.sub.1 K.sub.1 K.sub.2.sup.2               4                                                                   3rd       S.sub.T =S.sub.3 K.sub.1 K.sub.2 +S.sub.2 K.sub.1 K.sub.2.sup.2               +S.sub.1 K.sub.1 K.sub.2.sup.3                                      .                                                                             .                                                                             Nth       S.sub.T =S.sub.N K.sub.1 K.sub.2 +S.sub.N.sub.-1 K.sub.1                      K.sub.2.sup.2 + . . . S.sub.1 K.sub.1 K.sub.2.sup.N                 ______________________________________                                    

Since K₁ + K₂ = 1, the product K₁ K₂ is always less than unity, and asmay be seen, the contribution of the digital video signals from theearlier PRP's to the total digital sum signal appearing on the lines 44,is diminished as a result of successive cycles through the registers62-65 and multiplier 70. Since the moving target signal amplitude varieswith each PRP, any moving target signal component which is convertedthrough the low resolution A/D converter 38, is quickly attenuated onsuccessive cycles through the multiplication by the constants K₁, K₂ andthe addition on each cycle to successive PRP returns in which the targetsignal amplitude has changed. As a result, only the relatively fixedamplitude of the clutter signal component is represented in the digitalsum signals, and the process represents an implementation of a firstorder, or single pole digital low-pass filter. The implementation ofsuch a filter is derived through the use of Z transforms based upon theimpulse inputs of the digital signal provided by the A/D converter 38.The use of Z transforms and the derivation of the "pulse transferfunction" used to implement a digital filter equivalent of an analogfilter is described in a book entitled Sample Data Control Systems, byRagazzini and Franklin; McGraw Hill Book Company, Inc., 1958, pp. 64-79.

For a given pulse repetition frequency, the proper selection of the K₁,K₂ constant values (the sum of which is always unity) defines thebandwidth of the low-pass filter, the values being selected to providean upper corner frequency which is above that of the highest frequencyin the clutter frequency spectrum, which is subsequently dependent uponthe type of digital MTI and its operating environment. Typical decimalvalues for the constants may be K₁ = 0.02, and K₂ = 0.98.

The D/A converter 46 simultaneously converts the digital sum signals foreach successive PRP into an analog signal equivalent of the cluttercomponent for summation with the analog video signal on the line 24 inthe summing junction 50. The summing junction 50 subtracts the analogclutter signal on the line 48 from the total clutter plus moving targetvideo signal on the line 24, to provide a clutter plus moving targetsignal with a substantially greater target-to-clutter signal ratio thanthat of the analog video signal on the line 24, thereby reducing thedynamic range, or resolution accuracy requirement of the A/D converter28 while maintaining system target detection accuracy and reducingoverall cost.

The digital MTI radar system of the present invention permits the use oftwo A/D converters having lower dynamic range, and resolution accuracyrequirements and subsequent lower cost, together with a high resolution(low cost) D/A converter, in place of the high resolution, largerdynamic range A/D converter of the prior art digital MTI systems,therefore, providing for a reduction in signal processing costs. Thepresent invention further provides for the possibility of improvedsystem accuracy in detecting moving targets by using two A/D converterswhich individually have a lower resolution accuracy than the prior artsystem signal A/D converter, but which in combination provide an overallaccuracy which surpasses the prior art system, with little or noincrease in cost. The prefilter network of the present invention isseverally adaptable to any existing digital MTI system, therebypermitting the retrofit and subsequent cost reduction for existingsystems. Similarly, although the invention has been shown and describedwith respect to an illustrative embodiment thereof, it should beunderstood by those skilled in the art that the foregoing and variousother changes, omissions and additions in the form and detail thereofmay be made therein without departing from the spirit and the scope ofthe invention.

Having thus described a typical embodiment of our invention, that whichwe claim as new and desire to secure by Letters Patent is:
 1. A digitalmoving target indicator radar system, comprising:oscillator means forproviding a timing signal; signal means for providing pulsed radarenergy for illuminating a spatial sector at a pulse repetition period independence on the timing signal and for receiving, during a non-pulseportion of the pulse repetition period, reflected radar energy fromtargets within the sector, said signal means providing in responsethereto an analog video signal having a given amplitude, high frequencytarget signal component representative of moving targets and a largeramplitude, low frequency clutter signal component representative ofrelatively stationary targets, the analog video signal beingcharacterized by a given value target-to-clutter signal ratio; prefiltermeans, responsive to the analog video signal and the timing signal, forreducing the amplitude of the low frequency clutter component to providea low clutter analog video signal with a target-to-clutter signal ratiowhich is substantially greater than that of said signal means videosignal; first analog-to-digital conversion means, responsive to the lowclutter analog video signal and the timing signal for providing aplurality of low clutter digital video signals representative of theanalog video signal amplitude coincident in a plurality of successivetime intervals provided by the quantization of the pulse repetitionperiod in dependence on the timing signal, said analog-to-digitalconversion means having a resolution accuracy capable of providingamplitude discrimination between the target signal component and thereduced clutter signal component, the resolution accuracy beingsubstantially less than that required to provide equal accuracyamplitude discrimination of the target signal component of said signalmeans video signal; and filter and detection means, responsive to saidanalog-to-digital conversion means, for filtering the low clutterdigital signal to provide detection of the high frequency target signalcomponent, and for providing in response thereto a signal manifestationrepresentative of a moving target.
 2. The system according to claim 1,wherein said prefilter means comprises:second analog-to-digitalconversion means, responsive to the analog video signal and the timingsignal, for providing a plurality of successive digital video signalsrepresentative of the analog video signal amplitude coincident in eachof a plurality of successive, equal time intervals comprising thenon-pulse portion of the pulse repetition period, the time intervalsbeing provided by a quantization of the non-pulse portion in dependenceon the timing signal, said second conversion means having a resolutionaccuracy which is less than that required to provide accurate amplitudediscrimination between the target signal and clutter signal componentsof the analog video signal; digital low-pass frequency filter means,responsive to the digital video signals, for attenuating the highfrequency target signal component of the digital video signals toprovide a plurality of filtered digital signals representative of thelow frequency clutter component amplitude of the analog video signal;digital-to-analog conversion means, responsive to the plurality offiltered digital signals for providing an analog signal equivalentthereof, the analog signal being representative of a substantial portionof the clutter component amplitude of the signal means analog videosignal; and summing means, responsive to the analog signal from saiddigital-to-analog conversion means and to the analog video signal fromsaid signal means, for providing a sum analog signal representative ofthe difference amplitude therebetween, and for presenting the sum signalto said first analog-to-digital conversion means.
 3. The systemaccording to claim 2, wherein said digital low-pass frequency filtermeans comprises:first multiplier means, responsive to the digital videosignals for providing a plurality of filter input digital signals inresponse thereto, each of the input digital signals being representativeof the product of one of the plurality of digital signals and a scalingconstant value less than unity; full adder means, responsive at oneinput to the plurality of input digital signals, and responsive atanother input to a plurality of feedback digital signals, one for eachtime interval, for adding the input signal in each of the time intervalsto a corresponding time interval feedback signal to provide a pluralityof digital sum signals, each sum signal including a plurality of digitalbits presented by said adder means on a plurality of digital bit linesto said digital-to-analog conversion means; a plurality of serial shiftregister means, each connected for response to a different one of thedigital bit lines, each register means providing, in response tosuccessive time intervals, sequential shifting and storage of theplurality of successive digital bits on the respective bit line, eachbit corresponding to one bit from each of the plurality of successivedigital sum signals in each pulse repetition period, said plurality ofshift register means, in combination, storing the plurality of sumsignals in each pulse repetition period, and presenting the sum signals,one during each time interval, on a plurality of signal lines after adelay of one pulse repetition period; and second multiplier means,connected for response to said plurality of signal lines, formultiplying each of the delayed sum signals by a constant value lessthan unity, and for providing in response thereto, a plurality offeedback signals to said full adder means.
 4. An improved digital movingtarget indicator radar system of the type having an antenna, anoscillator for providing a timing signal, a transmitter for providing tothe antenna pulsed radar energy to the antenna having a pulse repetitionperiod in dependence on the timing signal for illuminating successiveportions of a spatial sector, a receiver for receiving, during anon-pulse portion of the pulse repetition period, reflected radar energyfrom targets within the sector and for providing in response thereto ananalog video signal having a small amplitude, high frequency targetsignal component from moving targets, and a high amplitude, lowfrequency clutter signal component from relatively stationary targets,the video signal being characterized by a small value target-to-cluttersignal ratio, an analog-to-digital converter for providing a digitalsignal equivalent of the analog video signal at a resolution accuracycapable of providing amplitude discrimination between the target signalcomponent and the clutter signal component, and filter and detectioncircuitry for providing a signal manifestation for moving targetsdetected within the spatial sector, wherein the improvementcomprises:first analog-to-digital conversion means, responsive to theanalog video signal and the timing signal, for providing a plurality ofsuccessive digital video signals representative of the analog videosignal amplitude coincident in each of a plurality of successive, equaltime intervals comprising the non-pulse portion of the pulse repetitionperiod, the time intervals being provided by a quantization of thenon-pulse portion in dependence on the timing signal, said conversionmeans having a resolution accuracy which is less than that required toprovide accurate amplitude discrimination between the target signal andclutter signal components of the analog video signal; digital low-passfrequency filter means, responsive to the digital video signals, forattenuating the high frequency target signal component of the digitalvideo signals to provide a plurality of filtered digital signalsrepresentative of the low frequency clutter component amplitude of theanalog video signal; digital-to-analog conversion means, responsive tothe plurality of filtered digital signals for providing an analog signalequivalent thereof, the analog signal being representative of asubstantial portion of the clutter component amplitude of said signalmeans analog video signal; summing means, responsive to the analogsignal from said digital-to-analog conversion means and to the analogvideo signal from said signal means, for providing a sum analog signalrepresentative of the difference amplitude therebetween, andrepresentative of a low clutter analog video signal with atarget-to-clutter signal ratio which is substantially greater than thatof the video signal from the receiver; and second analog-to-digitalconversion means, responsive to the low clutter analog video signal, forproviding a plurality of low clutter digital video signals, eachrepresentative of the analog video signal amplitude coincident in adifferent one of the plurality of time intervals of the pulse repetitionperiod, said second conversion means having a resolution accuracycapable of providing amplitude discrimination between the target signaland clutter signal components of the low clutter video signal, theresolution accuracy being substantially less than that required toprovide equal accuracy amplitude discrimination of the target signalcomponent of the receiver video signal, said second conversion meanspresenting the low clutter digital video signals to said filter anddetection circuitry.
 5. The improved system according to claim 4,wherein said digital low-pass frequency filter means comprises:firstmultiplier means, responsive to the digital video signals for providinga plurality of filter input digital signals in response thereto, each ofthe input digital signals being representative of the product of one ofthe plurality of digital signals and a scaling constant value less thanunity; full adder means, plurality at one input to the plurality ofinput digital signals, and responsive at another input to a plurality offeedback digital signals, one for each time interval, for adding theinput signal in each of the time intervals to a corresponding timeinterval feedback signal to provide a plurlaity of digital sum signals,each sum signal including a plurality of digital bits presented by saidadder means on a plurality of digital bit lines to saiddigital-to-analog conversion means; a plurality of serial shift registermeans, each connected for response to a different one of the digital bitlines, each register means providing, in response to successive timeintervals, sequential shifting and storage of the plurality ofsuccessive digital bits on the respective bit line, each bitcorresponding to one bit from each of the plurality of successivedigital sum signals in each pulse repetition period, said plurality ofshift register means, in combination, storing the plurality of sumsignals in each pulse repetition period, and presenting the sum signals,one during each time interval, on a plurality of signal lines after adelay of one pulse repetition period; and second multiplier means,connected for response to said plurality of signal lines, formultiplying each of the delayed sum signals by a constant value lessthan unity, and for providing in response thereto, a plurality offeedback signals to said full adder means.
 6. A method for detecting amoving target comprising the steps of:transmitting to an antenna apulsed radar energy at a pulse repetition period for illuminatingsuccessive portions of a spatial sector, and controlling the pulserepetition period with a timing signal; receiving reflected radarenergy, during a non-pulse portion of the pulse repetition period, fromtargets within the sector, and providing therefrom an analog videosignal having a high frequency target signal component from movingtargets, and a low frequency clutter signal component from relativelystationary targets; quantizing the non-pulse portion of each pulserepetition period into a plurality of equal time intervals in dependenceupon the timing signal; providing a plurality of digital video signalsfor each pulse repetition period, each digital video signal beingrepresentative of the analog video signal amplitude coincident in eachof the plurality of equal time intervals, at a resolution accuracy lessthan that required for amplitude discrimination of the high frequencytarget signal component from the low frequency clutter signal component;filtering the plurality of digital video signals to remove the highfrequency signal component, and to provide a filtered digital videosignal representative of the low frequency clutter signal component ofthe digital video signal; providing an analog signal equivalent of theplurality of filtered digital video signals in each pulse repetitionperiod, the analog signal being substantially representative of theclutter component of the analog video signal; summing the analog signalequivalent of the filtered digital video signal with the analog videosignal, to provide an analog sum signal representative of the differenceamplitude therebetween, and representative of a low clutter analog videosignal with a target-to-clutter signal ratio which is substantiallygreater than that of the received analog video signal; converting thelow clutter, analog video signal amplitude coincident in each of theplurality of equal time intervals into a low clutter digital signalequivalent at a resolution accuracy capable of providing amplitudediscrimination between the high frequency target signal and lowfrequency clutter components of the low clutter analog video signal;filtering the low clutter digital signal to extract only the highfrequency signal component; and detecting the amplitude of the extractedhigh frequency signal components.
 7. The method of claim 6, wherein thefiltering of the digital video signals comprises the steps of:providinga plurality of input digital signals in response to the plurality ofdigital video signals in each pulse repetition period, each of theplurality of input signals representing the product of a correspondingtime interval digital video signal and an input scaling constant valueless than unity; and combining each of the plurality of input digitalsignals from each pulse repetition period with one of a plurality offeedback digital signals coincident in the same time interval, toprovide a plurality of sum digital signals for each pulse repetitionperiod, each of the plurality of feedback signals being representativeof the product of an input digital signal from a corresponding timeinterval in the immediately preceding pulse repetition period and afeedback scaling constant having a value equal to the difference betweenthe input scaling constant value and unity, the plurality of sum digitalsignals representing the low clutter component of the digital videosignal.
 8. An improved method for detecting a moving target of the typeincluding the steps of transmitting pulsed radar energy at a pulserepetition period controlled by a timing signal, receiving reflectedradar energy and providing a composite analog video signal whichincludes a small amplitude, high frequency target signal component formoving targets, and a large amplitude, low frequency clutter signalcomponent from relatively stationary targets, converting the analogvideo signal to provide a digital video signal having a high resolutionaccuracy sufficient to insure amplitude discrimination of a highfrequency target signal component from the low frequency clutter signalcomponent, filtering the high resolution digital video signal to extractonly the high frequency signal component, and detecting the amplitude ofthe extracted high frequency digital signal component to provide asignal manifestation of a moving target for high frequency signalcomponent amplitudes in excess of a predetermined threshold, theimprovement comprising the steps of:quantizing the non-pulse portion ofeach pulse repetition period into a plurality of equal time intervals independence upon the timing signal; providing a plurality of digitalvideo signals for each pulse repetition period, each digital videosignal being representative of the analog video signal amplitudecoincident in each of the plurality of equal time intervals, at aresolution accuracy less than that required for amplitude discriminationof the high frequency target signal component from the low frequencyclutter signal component; filtering the plurality of digital videosignals to remove the high frequency signal component, and to providefiltered digital video signal representative of the low frequencyclutter signal component of the digital video signal; providing ananalog signal equivalent of the plurality of filtered digital videosignals in each pulse repetition period, the analog signal beingsubstantially representative of the clutter component of the analogvideo signal; summing the analog signal equivalent of the filtereddigital video signal with the analog video signal, to provide an analogsum signal representative of the difference amplitude therebetween, andrepresentative of a low clutter analog video signal with atarget-to-clutter signal ratio which is substantially greater than thatof the received analog video signal; and converting the low clutter,analog video signal amplitude coincident in each of the plurality ofequal time intervals into a low clutter digital signal equivalent at aresolution accuracy capable of providing amplitude discriminationbetween the high frequency target signal and low frequency cluttercomponents of the low clutter analog video signal.
 9. The improvedmethod of claim 8, wherein the filtering of the digital video signalscomprises the steps of:providing a plurality of input digital signals inresponse to the plurality of digital video signals in each pulserepetition period, each of the plurality of input signals representingthe product of a corresponding time interval digital video signal and aninput scaling constant value less than unity; and combining each of theplurality of input digital signals from each pulse repetition periodwith one of a plurality of feedback digital signals coincident in thesame time interval, to provide a plurality of sum digital signals foreach pulse repetition period, each of the plurality of feedback signalsbeing representative of the product of an input digital signal from acorresponding time interval in the immediately preceding pulserepetition period and a feedback scaling constant having a value equalto the difference between the input scaling constant value and unity,the plurality of sum digital signals representing the low cluttercomponent of the digital video signal.